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Re: assign_parm_setup_block handling of parallels

> First, some background.  I'm attaching my testcase.  As you can see,
> nominal_mode (SFmode) != passed_mode (DFmode).
> The multi-member optimization will nicely translate this:
> (parallel:DF (expr_list (reg:DI 3)))
> into
>   (reg:DI 3) -> pseudoDI -> subreg:DF(pseudoDI) -> float_truncate:SF(DF)
> which is exactly what we want, and gets flattened and joined back into
> the appropriate registers.

I see, thanks for the explanation.  So this would avoid going through the 
stack altogether?

> In the multi-member parallel optimization, we already check for
> incompatability between nominal_mode and passed_mode, and do the conversion
> appropriately.

You added this code, right?

> Although I think we should do the conversion in this case as well, I
> believe we will generate better code if we allow the multi-member
> parallel optimization.

Agreed.  Then the problem boils down to devising the best predicate.  I think 
the right criterion is that the optimization should avoid using the stack, 
because in this case it can only do a worse job than the rest of the code in 
assign_parm_setup_block (that was the pessimization on SPARC 64-bit).  Of 
course that's probably not very easy to write.

Eric Botcazou

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